In general, an electrophotographic image forming apparatus is detachably provided with a cartridge (i.e. developing agent container) containing therein toner (i.e. developing agent), and is configured to perform image forming by supplying the toner within the cartridge to a photosensitive body. In such an image forming apparatus, a pair of light transmission windows is oppositely provided on side walls of the cartridge, and an amount of toner within the cartridge is estimated on the basis of a light reception signal obtained by detecting light incident from one light transmission window through the other light transmission window.
However, the toner in the cartridge gradually deteriorates (i.e. the charge performance is gradually lowered) due to repeated agitation thereof, and thus if the number of printed sheets exceeds a predetermined number, it is hardly to obtain the initial charge performance, and the quality of an image being formed deteriorates. Accordingly, in order to form a favorable image, it is required that a predetermined amount of toner still remains in the cartridge without the whole amount of toner in the cartridge being consumed.
In the meantime, it is common that plural types of developing cartridges that contain different initial amounts of toner, for example, two types of cartridges including a large-capacity type developing cartridge and a small-capacity type developing cartridge, are commercially available. However, in the related-art image forming apparatus, it is determined that a cartridge replacement time has been reached when the amount of toner therein becomes not more than a predetermined amount regardless of the initial capacity of toner.
Since the toner deterioration state differs roughly depending upon the use time of the cartridge, even if the same residual amount of toner as that in the large-capacity type developing cartridge remains in the small-capacity type developing cartridge, the toner remaining in the small-capacity type developing cartridge has not deteriorated so much that it lowers the quality of an image. Accordingly, if it is determined that the cartridge replacement time has been reached when the amount of toner is not more than the predetermined amount regardless of the initial capacity of toner, it is determined that the cartridge is to be replaced in spite of the fact that the toner remaining in the small-capacity type developing cartridge is still tolerable to use, and thus it is not possible to efficiently use the toner,